1. Field of the Invention
The present invention relates to a developing roll for use in an electrophotographic apparatus such as a copying machine or a printer.
2. Description of the Art
In recent years, the melting point of toner for use in an electrophotographic apparatus such as a copying machine or a printer is lowered for fixing the toner instantly in response to demand for speed-up of copying or printing. Further, toner particles are made smaller in response to demand for high image quality in copying or printing, and also the toner has high electrostatic charge in response to the demand for prolonging life of copied or printed images. Since such toner-tends to have low resistance to stress, a soft developing roll has been recently popularly used for reducing stress imparted on toner.
Such a developing roll generally comprises a shaft, an elastic layer made of rubber or the like formed on an outer peripheral surface of the shaft and a coat layer (outermost layer) formed on an outer peripheral surface of the elastic layer (for example, see Japanese Unexamined Patent Publication No. 10-239985). In such a soft developing roll, the coat layer is provided with a function to form a uniform toner layer, as well as functions to be electrostatically charged and thus transfer the toner in cooperation with other material (s) for forming such a coat layer.
Among such functions, the function for transferring the toner is generally given by roughening the surface of the coat layer formed on an outer peripheral surface of the developing roll. There are various methods for roughening the surface. For example, there is the prevalent method that hard particles (sandy particles) such as urethane resin are dispersed in the coat layer of the developing roll so as to roughen the surface of the coat layer.
However, when the hard particles are dispersed in the coat layer, the surface hardness is increased due to the hard particles, which imparts stress on the toner. For this reason, the toner is easy to be deteriorated. If toner has less resistance to stress, the toner is easier to be deteriorated.
Further, the rough surface formed by such dispersion of the hard particles is usually composed of a combination of convex portions where hard particles exist and concave portions where hard particles do not exist. For this reason, if the mixing amount, density of dispersion and the like of the hard particles are not correctly controlled, the heights of the convex portions and the depths of the concave portions vary, so that variation of the surface roughness on the coat layer varies widely. Further, since the surface roughness changes with time due to cohesion of the hard particles, it is difficult to maintain a uniform surface roughness for a long time. Therefore, the developing roll in which the surface is roughened by the hard particles tends to have a non-uniform transportability of the toner and may affect image quality.
In view of the foregoing, it is an object of the present invention to provide a developing roll which may reduce stress imparted on the toner and have uniform transportability of the toner.